Higher quality has recently been required of crimped yarns, and therefore, in the spindle type false-twisting processing method most frequently adopted for the manufacture of crimped yarns, it is necessary to perform investigations and controls on the so-called three T conditions, namely the twisting tension, the heating temperature and the twist number. Among these conditions, the tension and temperature can be controlled relatively easily, but control of the twist number is very difficult. For example, in the case of a friction-driving type false twisting spindle, the rotation number is periodically investigated and amended so that the deviation of the rotation number is within a range of .+-. 1% prescribed rotation number. However, the control of the rotation number is very difficult owing to such troubles as sticking of oiling agents and dirts on the friction-driving portion, abrasion of said portion and elongation of a driving belt. For this reason, we adopted a fluid turbine-type false twisting spindle instead of the above friction-driving type false twisting spindle. However, also in this case, it was difficult to maintain a prescribed rotation number uniformly among a number of spindle units for a long time, because of the deviation of the processing precision in turbines and variation of the air pressure. Accordingly, at the experimental stage, the rotation number was detected with respect to each unit and control of the rotation number was effected with respect to each unit. However, provision of a detecting and controlling device in each unit is very expensive, though the yarn quality is not damaged, and hence, this method is not suitable for the practical operation.
It is therefore a primary object of the present invention to provide a false twisting machine which can give processed yarns having good quality without increase of the equipment cost.